M.M. van der Krogt

The effect of ankle foot orthosis stiffness on the energy cost of walking: A simulation study

BACKGROUND In stroke and multiple sclerosis patients, gait is frequently hampered by a reduced ability to push-off with the ankle caused by weakness of the plantar-flexor muscles. To enhance ankle push-off and to decrease the high energy cost of walking, spring-like carbon-composite Ankle Foot Orthoses are frequently prescribed. However, it is unknown what Ankle Foot Orthoses stiffness should be used to obtain the most efficient gait. The aim of this simulation study was to gain insights into the effect of variation in Ankle Foot Orthosis stiffness on the amount of energy stored in the Ankle Foot Orthosis and the energy cost of walking. METHODS We developed a two-dimensional forward-dynamic walking model with a passive spring at the ankle representing the Ankle Foot Orthosis and two constant torques at the hip for propulsion. We varied Ankle Foot Orthosis stiffness while keeping speed and step length constant. FINDINGS We found an optimal stiffness, at which the energy delivered at the hip joint was minimal. Energy cost decreased with increasing energy storage in the ankle foot orthosis, but the most efficient gait did not occur with maximal energy storage. With maximum storage, push-off occurred too late to reduce the impact of the contralateral leg with the floor. Maximum return prior to foot strike was also suboptimal, as push-off occurred too early and its effects were subsequently counteracted by gravity. The optimal Ankle Foot Orthosis stiffness resulted in significant push-off timed just prior to foot strike and led to greater ankle plantar-flexion velocity just before contralateral foot strike. INTERPRETATION Our results suggest that patient energy cost might be reduced by the proper choice of Ankle Foot Orthosis stiffness.

Self-paced versus fixed speed treadmill walking

Instrumented treadmills are increasingly used in gait research, although the imposed walking speed is suggested to affect gait performance. A feedback-controlled treadmill that allows subjects to walk at their preferred speed, i.e. functioning in a self-paced (SP) mode, might be an attractive alternative, but could disturb gait through accelerations of the belt. We compared SP with fixed speed (FS) treadmill walking, and also considered various feedback modes. Nineteen healthy subjects walked on a dual-belt instrumented treadmill. Spatio-temporal, kinematic and kinetic gait parameters were derived from both the average stride patterns and stride-to-stride variability. For 15 out of 70 parameters significant differences were found between SP and FS. These differences were smaller than 1cm, 1°, 0.2 Nm and 0.2 W/kg for respectively stride length and width, joint kinematics, moments and powers. Since this is well within the normal stride variability, these differences were not considered to be clinically relevant, indicating that SP walking is not notably affected by belt accelerations. The long-term components of walking speed variability increased during SP walking (43%, p<0.01), suggesting that SP allows for more natural stride variability. Differences between SP feedback modes were predominantly found in the timescales of walking speed variability, while the gait pattern was similar between modes. Overall, the lack of clinically significant differences in gait pattern suggests that SP walking is a suitable alternative to fixed speed treadmill walking in gait analysis.

Effects of adding a virtual reality environment to different modes of treadmill walking.

Differences in gait between overground and treadmill walking are suggested to result from imposed treadmill speed and lack of visual flow. To counteract this effect, feedback-controlled treadmills that allow the subject to control the belt speed along with an immersive virtual reality (VR) have recently been developed. We studied the effect of adding a VR during both fixed speed (FS) and self-paced (SP) treadmill walking. Nineteen subjects walked on a dual-belt instrumented treadmill with a simple endless road projected on a 180° circular screen. A main effect of VR was found for hip flexion offset, peak hip extension, peak knee extension moment, knee flexion moment gain and ankle power during push off. A consistent interaction effect between VR and treadmill mode was found for 12 out of 30 parameters, although the differences were small and did not exceed 50% of the within subject stride variance. At FS, the VR seemed to slightly improve the walking pattern towards overground walking, with for example a 6.5mm increase in stride length. At SP, gait became slightly more cautious by adding a VR, with a 9.1mm decrease in stride length. Irrespective of treadmill mode, subjects rated walking with the VR as more similar to overground walking. In the context of clinical gait analysis, the effects of VR are too small to be relevant and are outweighed by the gains of adding a VR, such as a more stimulating experience and possibility of augmenting it by real-time feedback.

European consensus on the concepts and measurement of the pathophysiological neuromuscular responses to passive muscle stretch

To support clinical decision‐making in central neurological disorders, a physical examination is used to assess responses to passive muscle stretch. However, what exactly is being assessed is expressed and interpreted in different ways. A clear diagnostic framework is lacking. Therefore, the aim was to arrive at unambiguous terminology about the concepts and measurement around pathophysiological neuromuscular response to passive muscle stretch.

Driving a musculoskeletal model with inertial and magnetic measurement units.

We developed and evaluated a new kinematic driver for musculoskeletal models using ambulatory inertial and magnetic measurement units (IMMUs). The new driver uses the orientation estimates based on sensor fusion of each individual IMMU and benefits from two important properties of musculoskeletal models. First, these models contain more complex, anatomical, kinematic models than those currently used for sensor fusion of multiple IMMUs and are continuously improved. Second, they allow movement between segment and measured sensor. For three different tasks, the new IMMU driver, (optical) marker drivers and a combination of both were used to reconstruct the motion. Maximal root mean square (RMS) joint angle differences with respect to the silver standard (combined IMMU/marker drivers) were found for the hip joint; 4°, 2° and 5° during squat, gait and slideboard tasks for IMMU-driven reconstructions, compared with 6°, 5° and 5° for marker-driven reconstructions, respectively. The measured angular velocities corresponded best to the IMMU-driven reconstructions, with a maximal RMS difference of 66°/s, compared with 108°/s and 91°/s for marker-driven reconstructions and silver standard. However, large oscillations in global accelerations occurred during IMMU-driven reconstructions resulting in a maximal RMS difference with respect to measured acceleration of 23 m/s2, compared with 9 m/s2 for reconstructions that included marker drivers. The new driver facilitates direct implementation of IMMU-based orientation estimates in currently available biomechanical models. As such, it can help in the rapid expansion of biomechanical analysis based on outdoor measurements.

Energy exchange between subject and belt during treadmill walking

Treadmill walking aims to simulate overground walking, but intra-stride belt speed variations of treadmills result in some interaction between treadmill and subject, possibly obstructing this aim. Especially in self-paced treadmill walking, in which the belt speed constantly adjusts to the subject, these interactions might affect the gait pattern significantly. The aim of this study was to quantify the energy exchange between subject and treadmill, during the fixed speed (FS) and self-paced (SP) modes of treadmill walking. Eighteen subjects walked on a dual-belt instrumented treadmill at both modes. The energy exchange was calculated as the integration of the product of the belt speed deviation and the fore-aft ground reaction force over the stride cycle. The total positive energy exchange was 0.44 J/stride and the negative exchange was 0.11 J/stride, which was both less than 1.6% of the performed work on the center of mass. Energy was mainly exchanged from subject to treadmill during both the braking and propulsive phase of gait. The two treadmill modes showed a similar pattern of energy exchange, with a slightly increased energy exchange during the braking phase of SP walking. It is concluded that treadmill walking is only mildly disturbed by subject-belt interactions when using instrumented treadmills with adequate belt control.

P038 Reproducibility of video screen measurement of sagittal joint and segment angles during gait in patients with spastic cerebral palsy

S72 Abstracts of the 17th Annual Meeting of ESMAC, Poster Presentations / Gait & Posture 28S (2008) S49-S118 Conclusions: Isometric strength can be reliably measured by a hand-held dynamometer in patients with cerebral palsy after MLS. Findings of this study indicate that early rehabilitation including strength training is recommendable. Introduction: Orthopaedic surgery aims at improving functional ability of children with CP, however it is known to initially reduce muscle strength, particularly in the presence of pre-existing weakness. Few studies have examined the progression of muscle strength following MLS in children with CP (1,2). This study aims at evaluating strength outcome and progression over time in this patient group after MLS. The assessment device for strength, a hand-held dynamometer, was evaluated for reliability and validity. Patients/Materials and Methods: Inclusion criteria for both studies were: hospitalization following MLS, diagnosis of spastic cerebral palsy and a GMFCS score of <4. For the reliability study, 28 children with CP (mean age 14 y2m; range 9 y6m-18 y10m) were recruited. Maximum isometric strength (M. Gastrocnemius, Mm. Vastii, M. Quadriceps, M. Hamstrings, M. Gluteus medius, M. Gluteus maximus) was evaluated by two testers using a portable hand-held dynamometer at two measurement points. To evaluate concurrent validity, muscle strength was also evaluated using the Manual Muscle Testing (MMT) of Daniels and Wothingham by the two testers. For the strength outcome study, strength measurements of 31 children (mean age 13 y11m; range 5 y9m-27 y5m) were performed before surgery, at the beginning of the rehabilitation program and at discharge. All children received conventional rehabilitation and a three times weekly strength-training program. Statistical analysis was carried out to assess the interrater reliability (Intraclass Correlation Coefficients) and concurrent validity of the dynamometer (Spearman rank correlation coefficients). To evaluate the progression of strength over time, Friedman tests were applied with post-hoc Wilcoxon signed rank tests. Results: The present study showed excellent reliability coefficients of strength measurements by use of a hand-held dynamometer in this patient group after MLS. Good to high correlation coefficients between the values of the dynamometer and those of the MMT supported the concurrent validity. Friedman tests indicated a highly significant difference in strength for all muscle groups between the three measurements (p < 0.0001). Post-hoc tests showed a highly significant decrease of strength for all muscle groups after surgery (p < 0.0001) and a highly significant increase of strength after the rehabilitation program for all muscle groups (p < 0.0001). At discharge, the strength of M. Gastrocnemius, Mm. Vastii and M. Quadriceps even reached its preoperative level. Discussion: The results of this study indicated a decrease in muscle strength after MLS, but also the possibility to increase strength already in the early rehabilitation phase. Some muscle groups even reached preoperative levels at discharge from the hospital. The more proximal muscle groups did not reach preoperative levels and probably need a longer training period to become stronger.

P 083 - Correlation between Oxford Foot Model kinematics and plantar pressure during voluntary varus/valgus gait in healthy children

Correlation between Oxford Foot Model kinematics and plantar pressure during voluntary varus/valgus gait in healthy children.

O 037 – Estimating musculotendon forces in children with cerebral palsy: The importance of the use of electromyography in neuromusculoskeletal modelling

1. Introduction Computational modelling of the neuromusculoskeletal system (NMSS) can potentially provide detailed insight into muscle function to optimize treatment planning and evaluation in cerebral palsy (CP). Commonly, static optimization is used to solve the redundancy problem in estimating muscle forces, assuming, for example, minimization of the muscle activation squared. However, since the primary problem in children with CP is an aberrant motor control [1], it is questionable if using this criterion is applicable in this clinical population. Electromyography (EMG) might be used to further inform optimization procedures and improve model performance. 2. Research question What is gained in using EMG-based NMSS modelling to estimate muscle activations during gait in children with CP compared to static optimization? 3. Methods Five patients with CP participated (11.4 ± 2.7 y, GMFCS I–II). Each patient walked on an instrumented treadmill at comfortable self-selected walking speed, while marker trajectories, ground reaction forces and EMG of eight lower limb muscles were recorded. OpenSim [2] was used to scale a generic model to the lengths of the patients’ segments, using static pose data. Inverse kinematics and inverse dynamics were applied on this model for four randomly selected strides. Four different modeling approaches were used to estimate musculotendon forces. The first two approaches used the static optimization tool in OpenSim to estimate muscle forces, either directly (SO), or after scaling the maximum isometric force of each muscle to body mass (SO_MIF). The third (EA_uncal) and fourth (EA_cal) approach used EMG-assisted modelling, combining EMG data with static optimization (CEINMS software [3]). In EA_cal musculotendon parameters were first calibrated to optimize the matching of EMG-driven moments with inverse dynamics moments [4]. To examine each model’s performance, predicted activations were correlated to activations derived from EMG, and predicted joint moments were correlated to those estimated using inverse dynamics. Correlations were quantified by R^2, and compared between approaches with repeated measures ANOVA. 4. Results Scaling the maximum isometric force did not significantly improve the R^2 for the model’s activations with EMG activations (Fig. 1A). Using an EMG-assisted approach improved the R^2, but not significantly for EA_uncal. EA_cal showed a significantly higher R^2 vs. SO (p = 0.050) and EA_uncal (p = 0.022). Using both EMG-assisted approaches only slightly decreased the tracking of inverse dynamics moments (Fig. 1B), without any significant differences.

Optimal calibration of instrumented treadmills using an instrumented pole.

Calibration of instrumented treadmills is imperative for accurate measurement of ground reaction forces and center of pressure (COP). A protocol using an instrumented pole has been shown to considerably increase force and COP accuracy. This study examined how this protocol can be further optimized to maximize accuracy, by varying the measurement time and number of spots, using nonlinear approaches to calculate the calibration matrix and by correcting for potential inhomogeneity in the distribution of COP errors across the treadmills surface. The accuracy increased with addition of spots and correction for the inhomogeneous distribution across the belt surface, decreased with reduction of measurement time, and did not improve by including nonlinear terms. Most of these methods improved the overall accuracy only to a limited extent, suggesting that the maximal accuracy is approached given the treadmills inherent mechanical limitations. However, both correction for position dependence of the accuracy as well as its optimization within the walking area are found to be valuable additions to the standard calibration process.